Conductive ink choices
Conductive inks are typically made of metallic particles such as silver or copper flakes in a retaining matrix, or carbon flakes/particles in a retaining matrix. Carbon is typically 2 magnitudes less conductive compared to metals but cheaper. Traditionally, the matrix was a ceramic such as glass frit, but now increasingly it is a polymer (known as Polymer Thick Film, PTF).
The retaining matrix is not conductive or weakly conductive - once printed the matrix needs to be reduced so that conductance through the material occurs by conductive particles in contact with each other, which is done by curing. Curing can be performed by UV or temperature, for example. Ceramic mixtures need high cure temperatures, such as 650 degrees Celsius for several minutes. This results in the need for a tough and expensive substrate. PTF mixtures have lower cure requirements - such as 150 degrees Celsius and can therefore use cheaper substrates such as PET. However, curing temperatures depend on the ink forumlation; some dry quickly at room temperature which is sufficient for some applications, such as the RFID tag antennas at UHF (about 900MHz). Different ink formulations mean that curing is just one factor in the final bulk conductivity of the printed ink.
Applications of conductive inks
Carbon based inks, which are only weak conductors, have typically been used for EMI/RF shielding, such as on monitor screens and speakers.
Metallic based inks are commonly used for membrane switches and circuits, and now increasingly for RFID tag antennas. In the case of RFID, the conductivity of metal particle inks is typically more than sufficient so curing can be done at relatively low temperatures for a few seconds - using a heated press is sufficient. Other applications include the use of conductive ink for connections on smart blister packs, which record when a pill is popped (because a circuit is broken), and for tamper evidence packs which work on a similar principle. As another example, tens of millions of printed battery testers have been sold, based on a conductive ink. Read a recent article from Printed Electronics Review on this.
Ink allied to substrate
Similarly to inks in consumer inkjet printers, conductive ink formulations and effectiveness are strongly related to the substrate type. Inks printed on substrates they are not designed for, even the difference between, for example, coated papers to normal papers, can display very different conductivity properties. Companies such as Precisia and DuPont are developing a range of inks suitable for different print processes and substrates.
Epoxies Etc, based in the US, have just announced a new ink which is effective on a variety of substrates. This can be screen printed or sprayed. The inks can be deposited to substrates such as kapton, mylar, glass, polyester and ceramic.
Together with printed transistors and components, printable conductive inks will be needed for the interconnections between these components and basic components such as sensors, capacitors and resistors.